Once the model has been rigged properly and has NO errors, you select all the body parts one by one, clone them, go in sub-object mode and invert the normals. Then you add a push modifier (and move it below the skin modifier) add desired value for outline thickness.

Once that's done, you have to rename the body parts, example: rename the cloned torso to "torso_outline" and link this mesh to the torso, do the same for "l_arm" which becomes "l_arm_outline" and linked to "l_arm" and so on until you linked all the clones.

Last step is to write a shader that will make the texture 100% illuminated, and shade the model with a .TGA gradient. Apply a 100% black texture to your outline objects.